In the case of a vehicle equipped with a manual transmission, the vehicle is normally parked to hold the vehicle immovable with a combined application of the transmission shifted to a lower speed such as the first speed to lock the rotation of drive wheels, and a parking brake.
In the case of an electric vehicle and a vehicle equipped with an automatic transmission, the above-mentioned method for the rotation lock of the drive wheels at the time of parking is not feasible. Therefore, there is provided a park lock device for the rotation lock of the drive wheels. During parking, the vehicle is held immovable by application of the park lock device and the parking brake.
As the park lock device, there has been recently proposed, and employed in some vehicles, a so-called by-wire control type park lock device which has a construction to operate a park lock mechanism with a park lock actuator such as a motor, and which is arranged to convert a parking command generated by a driver's shift operation, into an electric signal and to operate the actuator in accord with the electric parking command.
In the case of the electric vehicle, the by-wire control type park lock device is indispensable in relation to the control system of the entire vehicle.
In such a by-wire control type park lock device, a power supply or power source is essential to operate a park lock actuator, and to drive a controller for controlling the park lock actuator.
Therefore, at the time of power supply failure such as inability of charging the power supply of the park lock device, or inability or malfunction of charging sufficiently, the electric storage state of the park lock power supply becomes worse gradually. Consequently, the park lock device becomes unable to be operated properly in time, and the driver is obliged to park the vehicle in the state without the rotation lock, to the disadvantage of the safety.
As is often the case with the electric vehicle, in the case of an electric parking brake arranged to be operated through an actuator by driver's operation of a nearby switch, unlike a mechanical parking brake operated by the driver mechanically, the electric power source is shared by the electric parking brake and the park lock device in a common-sense arrangement. Therefore, there arises a problem that it becomes impossible to set the vehicle in the parking state since the parking brake becomes inoperative simultaneously at the time of an inoperative state of the park lock device.
Therefore, at the time of a failure of the park lock power supply, there is a demand for finish operation of the park lock device at least before the park lock power supply falls in a state unable to operate the park lock device. However, the inventors of this application are not aware of technique to fulfill this demand.
A patent document 1 shows a following proposal of a countermeasure technique to an abnormal condition in a shift range changeover device. In the case of occurrence of an abnormal condition in a shaft range changing drive system in a by-wire control type shift range changeover device for an automatic transmission, the technique of this patent document proposes to inhibit an electric current control to the shift range changeover drive system in the abnormal state and to operate the shaft range changeover mechanism only with a torque produced by supplying electric current to a shift range changing drive system remaining in the normal state free of an abnormality.
However, the abnormal time countermeasure technique disclosed in the patent document 1 is not a measure against a failure in an electric power supply, but a measure against a failure in the drive system. Therefore, the technique of the patent document 1 is not effective as measures against a failure in a park lock power supply treated by this invention.
If the concept of the abnormal time countermeasure technique of the patent document 1 were used for measures against a failure in the park lock power supply treated by the present invention, a resulting system would be constructed to have two park lock power supply systems having two park lock power supplies, respectively, and to operate the park lock device by inhibiting the supply of power from the park lock power supply involving an abnormal condition and supplying electric power only from the park lock power supply in the normal state free from abnormality.
The arrangement including two of the park lock power supplies and forming two of the park lock power supply systems is capable of operating the park lock device with a normal one of the two park lock power supply systems even if one of the two park lock power supply systems fails. However, this arrangement increases the cost, specifically because of the cost of the costly park lock power supplies.